1. Field of the Invention
This invention relates to a heat sink material used for semiconductor devices, especially to a cladding material used for a heat sink on which LSIs, ICs, power transistors are mounted and are operated with high power, and also relates to its manufacturing method, to a press forming method for the cladding material, and to a heat sink using the cladding material.
2. Description of the Related Art
For heat sinks on which semiconductor devices, especially LSIs, ICs, power transistors operated with high power, are mounted, efficient dissipation of heat generated by these semiconductor devices is required.
Therefore, high mechanical strength and high thermal conductivity are required of these heat sinks. On the other hand, materials with low thermal expansion coefficient as lower than or equal to 6×10−6/K, such as silicon which constitutes a semiconductor chip, ceramics material such as alumina which constitutes a semiconductor chip, or Kovar® etc., are bonded and used. Because this bonding temperature is about 850° C., when these are cooled after these materials are bonded so that no camber is made at this temperature, the heat sink will be contracted greatly compared with these bonded materials. Therefore, at room temperature, heat distortion or camber is generated in this heat sink. In order to reduce these, the thermal expansion coefficient of this heat sink is needed to nearly equal to that of bonded materials, or to be low.
Under these circumstances, single metal such as Cu (copper), Mo (molybdenum), and W (tungsten) etc., and composite metals such as Cu—W, and Cu—Mo etc., have been used as a material of the heat sink.
Among these materials, Cu has high thermal conductivity around 390 W/(m·K), although the thermal expansion coefficient of this is as high as 20×10−6/K.
On the other hand, Mo and W have low thermal conductivity (Mo:5×10−6/K and W:4×10−6/K) respectively, although thermal expansion coefficient is lower, 142 W/(m·K) and 167 W/(m·K) respectively, compared with Cu. Therefore, the cladding material which is composed of lamination structure, in which a material such as Mo or W with low thermal expansion coefficient and with high thermal conductivity, and another material such as Cu is combined, is used.
For example, the Cu/Mo/Cu cladding material is shown in patent document 1 (JP,2-102551), A as a heat sink which consists of 3-layered structures. Here, by varying the volume ratio of Mo in the cladding material of this 3 layered structure in the range of 20% to 99.6%, then thermal conductivity and thermal expansion coefficient are controlled, and both higher thermal conductivity than Mo alone and lower thermal expansion coefficient than Cu alone, are obtained. In patent document 2 (JP, 6-268115,A), the relation between the thermal expansion coefficient of the cladding material with 3 layered structure of Cu/Mo/Cu and the volume ratio of Cu, is shown. In the cladding material of this composition, when the number of used Mo layer is one, in order to set the thermal expansion coefficient below 12×10−6/K for example, the amount of used Mo with low thermal conductivity must be made higher than or equal to 20% of the whole mass. Therefore, the thermal conductivity in the thickness direction of this cladding material may be 230 W/(m·K) at most.    [Patent documents 1] JP,2002-201075,A    [Patent documents 2] JP,2005-26252,A
However, recent semiconductor device becomes high powered, and on the material of heat sinks on which these devices are mounted, low thermal expansion coefficient and higher thermal conductivity are needed. For this requirement, thermal conductivity of above mentioned cladding materials is not enough. Therefore, material with thermal conductivity as high as Cu alone and also with low thermal expansion coefficient, is needed.
In the above-mentioned cladding material, in order to make thermal expansion coefficient low, the volume ratio of Mo layer is needed to be lower than or equal to 20%. Mo is harder than Cu and hard to be patterned mechanically, therefore, the workability of the cladding material with large volume ratio of Mo gets worse, and fabrication using press punching method etc., which is suitable press forming method for mass production, is hard to be applied. Therefore, other press forming methods at high cost are required for this patterning, therefore, products using this cladding material becomes expensive. Therefore, it was difficult to obtain a cladding material with both high thermal conductivity and low thermal expansion coefficient at low cost.
This invention is made to solve these problems, and the purpose is to offer a cladding material which solves the above-mentioned problems, its manufacturing method, and a press forming method for the cladding material, and to offer a heat sink with both high thermal conductivity and low thermal expansion coefficient using this.